This invention relates in general to the controlling of nitrogen oxide (NOx) emissions from fossil-fuel power plants. More particularly, the invention relates to a catalyst composition for controlling nitrogen oxide emissions from fossil-fuel power plants by selective catalytic reduction (SCR) with ammonia using pillared clay catalysts.
Nitrogen oxides (NO, NO2, and N2O, collectively referred to as xe2x80x9cNOxxe2x80x9d) present in the exhaust gases from combustion of fossil fuels, continues to be a major source for air pollution. Nitrogen oxides contribute to photochemical smog, acid rain, ozone depletion and greenhouse effects. As a major cause of acid rain, nitrogen oxides additionally contribute to the pollution of water and soil. The current technology for reducing nitrogen oxides emissions from power plants is by selective catalytic reduction (SCR) of NOx (where x is 1 to 2) with ammonia in the presence of oxygen, to form nitrogen and water. The overall reaction is as follows:
4NH3+4NO+O2xe2x86x924N2+6H2O
Many catalysts have been reported to be active for this reaction, such as vanadia and other transition metal oxides (e.g., V2O5, CuO, Fe2O3, Cr2O3, Fe2O3xe2x80x94Cr2O3, Nb2O5, and the like), pillared clays and zeolite-type catalysts. For example, in the SCR reaction, a vanadia catalyst V2O5+WO3 (or MoO3) supported on TiO2 is commonly used as commercial catalysts. The mechanism of the reaction on the vanadia catalysts has been studied extensively and is reasonably understood although several different mechanisms have been proposed. Although the SCR technology based on vanadia catalysts has been commercialized, problems with this approach still remain. For example, the vanadia catalyst promotes high activity for the oxidation of SO2 to SO3, promotes formation of N2O at high temperatures, and is toxic. The formation of SO3 is undesirable because it reacts with NH3 and H2O to form NH4HSO4, (NH4)2S2O7 and H2SO4 which cause corrosion and plugging of the reactor and heat exchangers in the power plant. Hence there are continuing efforts in developing new catalysts.
Pillared interlayered clay (PILC) is a unique two-dimensional zeolite-like material known in the art. Because of its large pores, pillared clays have been evaluated as a possible replacement for zeolite as the catalyst for fluid catalytic tracking (FCC) which operates at near 700xc2x0 C. with high steam concentration. However, pillared clays were found not to be suitable due to excessive carbon deposition and the limited hydrothermal stability of the pillared clay structure. Pillared clays have also been studied for catalyzed alcohol dehydration, alkylation and other acid catalyzed reactions. Pillared clays have also been evaluated for selective catalytic reduction of NOx, as described in U.S. Pat. No. 5,415,850. Specifically, the ""850 patent discusses pillared clay composition doped with certain concentrations of certain metals. While such reported pillared clay catalysts are useful, there is a need in the industry to continue development of catalysts that exhibit improved catalytic activity for selective catalytic reduction of NOx.
Accordingly, it is an object of the present invention to provide an improved catalyst composition for reducing nitrogen oxide emissions.
In summary, the present invention described herein are metal ion exchanged pillared interlayered clay catalysts. More particularly, the present invention provides a catalyst for selective catalytic reduction of nitrogen oxide compounds with ammonia comprising a composition of one or more pillared interlayered clays, one or more metal ions exchanged with the pillared interlayered clays, and one or more promoter ions exchanged with the pillared interlayered clays. Specifically, the pillared interlayered clay (PILC) catalysts of the present invention include Fe3+, Cr3+, Mn2+, Co2+, Cu2+, and Ni2+ exchanged Al2O3-PILC, TiO2-PILC, ZrO2-PILC, SiO2-PILC and Fe2O3-PILC catalysts. These ion-exchanged PILCs are further promoted by rare earth metals, such as Ce, La, Pr, Tb and Nd. These pillared clay catalysts of the present invention show remarkable activity for the SCR reaction. Of significant advantage, as compared to prior art commercial catalysts, the pillared clay catalysts of the present invention yield higher activity, reduce the SO2 oxidation of SO3 by up to about 85%, and yield substantially less undesirable N2O.